As a wind power generation system, the horizontal-axis wind turbines are widely used on land. Countries with matured wind turbine market have faced shortage of sites suitable for installing wind turbines with sufficient wind energy. Hence, in such countries, it is necessary to install wind turbines offshore where stable wind force can be obtained and large areas are available. However, as of now, wind turbines have been installed offshore only by a method in which, as in the case on land, a wind turbine is installed on foundation onto a seabed in a sea area near a coastline with an extremely shallow water depth of about 10 m.
Since there is an expectation for further increase of the offshore installation in the future, development of a practical method for installing a wind turbine as a floating structure is demanded. Since electric power is generally required on land, the electric power has to be supplied to land through electric wires. To reduce loss during the transmission, the wind turbine has to be installed near land, and has to be installed in a shallow sea area. For a wind power generation system having a floating structure, which is expected as a next-generation offshore wind turbine installation method, a method is first desired which enables economical installation in a shallow sea area with a water depth of about 20 to 30 m.
When a wind turbine converts wind energy to rotation force, the wind turbine receives a strong wind force. The strong wind force generates a moment which causes the wind turbine to turn over. The horizontal-axis wind turbine, which is developed on land, receives the wind force at one point by a horizontal shaft supported at a high position in the air. Hence, a huge overturning moment is generated at the root of a vertical support column of the horizontal-axis wind turbine. In the horizontal-axis wind turbine, the wind turbine is attached to rotate around the vicinity of an upper end of the wind turbine support column, and the wind turbine has to continue to change its orientation so that the wind turbine can always face the wind. Hence, it is impossible to provide guy-wires for supporting the support column in order to receive the above-described huge moment. Accordingly, the support column of the horizontal-axis wind turbine has to be fixed to the ground as firm as possible, and it is difficult to rotate the wind turbine together with the support column to change the direction of the wind turbine. If a turntable was provided on the ground level, the overturning moment of the support column cannot be received, unless the diameter of the turntable is excessively increased. For this reason, in general, the turntable of a horizontal-axis wind turbine is provided immediately below a nacelle provided in an upper end of the support column. Meanwhile, to provide functions necessary for a horizontal-axis wind power generation, it is necessary to provide devices, such as a horizontal-axis bearing support system, a step-up gear, a power generator, a brake, and a blade pitch control device, around the rotation axis of the wind turbine. These devices are desirably provided closer to the wind turbine than the turntable, in order to avoid fluctuation in the rotation torque and interference with the rotation of the turntable. Not only all these major devices, but also peripheral devices including a lubricant oil system, a control panel, and the like are provided in the nacelle in the air. Consequently, the center of gravity of the horizontal-axis wind turbine is located at an extremely high position. In addition, when the horizontal-axis wind turbine is attached firmly to a floating structure, rolling centered at the floating structure is amplified at the upper end of the support column, and then an excessive lateral G force is generated. Hence, it is disadvantageous that the devices disposed in the nacelle have to have strengths, lubrication systems, and the like for withstanding such lateral G force.
FIG. 17 schematically shows, as Comparative Example 1, a relationship between inclination and stability moment in a case where a horizontal-axis wind turbine is placed on a floating structure.
In general, in order for a floating structure to have a stability moment, the center of gravity needs to be at a position lower than the metacenter (the intersection of the buoyancy line and the center line of the floating structure) located near the floating structure. In a horizontal-axis wind turbine 200 configured as described above, heavy devices are all located at high positions in the air, and hence the center of gravity G is so high that the horizontal-axis wind turbine 200 cannot have stability moment. Suppose a case where the horizontal-axis wind turbine 200 of a construction as provided on land is installed by fixation to a floating structure 201. In such a case, even if the inclination of the floating structure 201 is slight, the gravity force F1 acts outside the buoyancy F2 acting on the floating structure 201, because of the high center of gravity G as shown in FIG. 17. Hence, a force acts to further incline the floating structure 201. Moreover, the floating structure 201 receives a huge and fluctuating overturning moment, because of a wind force F3 received at a high position as shown in FIG. 17.
In other words, since the floating structure 201 does not have a necessary stability moment, and receives a huge and fluctuating overturning moment because of the wind force F3, there is a problem that such a structure is impractical as a floating structure.
To solve these problems, it is necessary to provide all the major devices at low positions on the floating structure, so that the center of gravity G and work areas for the maintenance are lowered as much as possible.
In the case of the horizontal-axis wind turbine 200, the turntable has to be disposed at an upper end of the wind turbine support column 202, unless the necessity for the firm fixation of the wind turbine support column 202 to the floating structure 201 as seen in the example of the land wind turbine earlier can be eliminated. Consequently, all the upstream devices are placed in the nacelle 203 above the turntable, and hence it is difficult to lower the center of gravity G.
FIGS. 18(a)-(c) schematically show, as Comparative Example 2, a relationship between inclination and stability moment in a case where a vertical-axis wind turbine is placed on a floating structure, where FIG. 18(a) shows a state with a slight inclination, FIG. 18 (b) shows a state with an increased inclination, and FIG. 18(c) shows a state with a further increased inclination.
In contrast to the horizontal-axis wind turbine 200 of Comparative Example 1, the center of gravity G of a vertical-axis wind turbine 300 as shown in FIGS. 18(a)-18(c) should be lowered to a great extent, because all heavy devices can be provided not high in the air but on the floating structure 301 as in the case of the ground where the heavy devices are provided on a base in general. However, as seen in an example on land, in a case of a vertical-axis wind turbine 300 in which the support column 302 itself rotates with a rotor, it is difficult to fix the support column 302 in such a manner as to withstand an overturning moment due to a wind force F3, and it is necessary to provide guy-wires (not illustrated) in four directions to support an upper end of the support column 302. This necessitates a floating structure having a wide deck surface not smaller than a size necessary for a buoyant body. In addition, aside from the problem of the guy-wires, the lowering of the center of gravity to this extent causes the following problem. Specifically, when the inclination of the floating structure 301 due to the wind force F3 or the like is small as shown in FIG. 18(a), a stability moment is exerted because the amount of the lateral shift of the buoyancy center C is larger than the amount of the lateral shift of the center of gravity G by the inclination. As the inclination further increases, as shown in FIG. 18(b), the lateral shift of the center of gravity G eventually becomes equal to the lateral shift of the buoyancy center C, and the stability moment is lost. With further inclination, a force to cause further inclination acts as shown in FIG. 18(c). To put it differently, there is a problem that the stability moment is lost and the floating structure 301 is overturned, when the inclination angle exceeds a certain value. This is a phenomenon occurring because of the following reason. Specifically, when the center of gravity G is located above the floating structure 301, the center of gravity G is shifted laterally, as the inclination increases. Here, since the buoyancy center C cannot be located outside the floating structure, the lateral shift of the center of gravity G exceeds the lateral shift of the buoyancy center C. This problem is unavoidable, unless the center of gravity G is located not higher than the waterline of the floating structure 301.
FIG. 19 schematically shows, as Comparative Example 3, a relationship between inclination and stability moment in a case where a vertical-axis wind turbine is supported to be incapable of tilting with respect to the floating structure, and a ballast is provided in water.
For an ordinary yacht, a stability system has been achieved in which a ballast is provided in water so that a stability moment is exerted with any inclination. By applying such a stability system of a yacht, a vertical-axis wind turbine 400 is conceivable in which, a support column 403 is supported to be incapable of tilting with respect to a floating structure 401, and a ballast 402 is provided in water, as shown in FIG. 19. The vertical-axis wind turbine 400 can be achieved because the center of gravity G is lower than the rotation center (buoyancy center C) of the inclination movement in the vicinity of the floating structure 401. However, in this form, an excessive stress is placed on a joint part 401a of the support column 403 to the floating structure 401, and hence it is impractical to support the support column 403 by the joint part 401a alone. This form can be achieved only when wires (not illustrated) called forestay or sidestay supporting the support column 403 are provided in three or four directions, as in the case of the guy-wires of the vertical-axis wind turbine on land. In addition, when this structure is directly applied to a wind power generation system operated while moored offshore, operators are exposed to danger because the floating structure 401 is greatly inclined with the support column 403. In addition, the load on the mooring system which is influenced by the inclination of the floating structure 401 excessively increases particularly in shallow areas.
Various methods have been studied for overcoming the insufficiency in the stability moment of such a floating structure. Examples of proposed methods include a method in which multiple horizontal-axis wind turbines are all disposed on a single huge floating structure; a method in which multiple horizontal-axis wind turbines are disposed and floating structures supporting the horizontal-axis wind turbines, respectively, are rigidly joined to each other (see, for example, Patent Document 1); a method in which the stability is obtained by using a floating structure, called a spar, having a cylindrical shape elongated in the longitudinal direction and extending deep under the water (see, for example, Patent Document 2), a method called TLP in which a floating structure is stabilized by being pulled toward the seabed by metal pipes called tendons or the like (see, for example, Patent Document 3); and the like.
However, each of the methods has such a drawback that the size of the floating structure is too large relative to the amount of energy harvested by the system from wind force, and hence the construction costs and the installation costs are too much, which make the method economically impractical. Moreover, each of the methods is based on a concept in which a certain water depth is necessary, considering the change in draft of the huge structure due to rolling, the draft of the vertically elongated structure, the geometric movement range of the tendons pulling in the longitudinal direction, and the like. Hence, these methods have such a drawback that these methods are unsuitable for installation in shallow areas near land where the electric power is required as mentioned above.